In orthotropic box girders, the bottom and top surfaces of the deck act as flanges and the side walls as twin webs, giving the structure overall longitudinal strength.
Transverse diaphragms at intervals along the bridge help stiffen the structure.
To prevent the deck bending locally under vehicle loading, longitudinal trough section steel stiffeners are welded to the underside of the deck. While it is possible to get into the box girder to paint and carry out maintenance, the insides of the trough section stiffeners are inaccessible.
Problems are particularly acute at the joints between longitudinal trough section stiffeners and the underside of the deck. Connections were typically made with fillet welds which, because of the difficulty in welding from inside the trough, were from one side of the joint only, and generally did not achieve much penetration.
As larger numbers of heavier trucks thunder across Germany's bridges, transverse deck deformation is becoming much more acute and more frequent - many thousands of cycles a day. Each deformation momentarily distorts the top plate of the deck and the stiffeners attached to it (see diagram).
Distortion ntroduces torsion into the welds between stiffeners and plate - something they were not designed for. Cracks develop in the brittle welds, and are often propagated in the top plate.